Basically, you have to call and ask (or nominate us to do that for you!), although the bigger casino companies have chain-wide affiliations. Although it may well be more convenient to rent from the rental hub at McCarran, not only do you incur a "customer facility fee" ($3/day or more) if you rent at the airport, but in our experience, you're likely to pay a higher daily rental charge there, too (we know not why). For our epic three-part QoD on pretty much all you need to know or consider when renting a car (shopping around, insurance, fees, accidents, damage, breakdowns, valet parking -- the whole nine yards) see QoD Archives 10/28/14-10/30/14).
Meanwhile, here’s how it breaks down (no pun intended) among the Las Vegas casinos that provide rental-car service:
In the cases of Green Valley Ranch and Red Rock Resort, each hotel receives a quota of rental cars from its provider(s), which it then subleases to hotel guests. We hope this answer has made renting a car in Las Vegas a bit easier for you.